The scroll type fluid displacement apparatus has two scrolls. Each scroll includes an end plate and a spiral wrap. The wrap is attached to the end plate and includes two flank surfaces and an axial tip. The wrap which is found in many scroll type machines is an involute spiral. The two scrolls are positioned relative to each other so that the wraps and the end plates cooperate to form one or more fluid chambers. The scrolls are driven in an orbital path relative to each other to move the fluid in the chambers. If the orbital movement moves fluid from the central portion of the scrolls toward the radially outer edge, the fluid is generally being expanded. If the orbital movement moves fluid from the radially outer portion of the scrolls toward the central portion of the scrolls, the fluid is generally being compressed or pumped.
Two scrolls can be rotated about parallel axes that are displaced from each other to obtain the required orbital movement relative to each other. Relative orbital movement can also be obtained by holding one scroll in a fixed position and driving the other scroll in an orbital path.
Scroll type fluid displacement machines often employ scrolls that form at least one pair of fluid pockets. The fluid pockets are bound by flank surfaces of the wraps and by surfaces of the end plates. For these machines to efficiently compress, pump or expand fluids, the fluid pockets must be sealed to reduce fluid leaks. The flank surfaces of the wraps are smooth and accurately shaped so that contact between them will at least limit fluid leaks. The axial tips of the wraps and the surface of the end plates can also be accurately shaped to limit fluid leaks when the scrolls are at a uniform temperature.
During pumping, expansion or compression of fluids, the fluid temperature changes. The change in fluid temperature as the pressure changes in the fluid pockets changes the temperature of the scrolls. The temperature change generally results in the temperature at the center of the scrolls being higher than the temperature at the radially outer edges of the scrolls. This temperature variation across the scrolls results in variations in the axial height of the wraps due to thermal expansion and contraction. The variation and change in height of the wraps must be accommodated to prevent leakage of fluid between the axial tips of the wraps and the surfaces of the end plate. Axial expansion of the wraps can result in the axial tips of the wraps exerting sufficient force against the adjacent scroll end plate to cause excessive wear and frictional load.
Fluid leakage between the wrap flank surfaces of one scroll and the wrap flank surfaces of another scroll is controlled by contacts between the flank surfaces. The flanks of scroll wraps are accurately formed, their surfaces are smooth and may even have special coatings. The drive systems employed are designed to maintain contact between the flank surfaces during orbital movement of a pair of scrolls relative to each other without causing wrap damage.
Fluid leakage between the axial tips of the wraps and the surfaces of the end plates is controlled by axial tip seals. Axial tip seals have been used which are resilient or spring biased so that the seals accommodate thermal expansion and contraction of the wraps and remain in sealing contact with the surfaces of the end plates. Seals which are free to float in a tip seal groove have also been used. The fluids in the fluid pockets hold the floating seals in sealing contact with the surfaces of the end plates during operation.
Axial tip seals are held in a tip seal groove or groove sections in the axial tip of each scroll wrap. A tip seal groove can not extend all the way to the end of a wrap in the center portion of a scroll or to the radially outer end of a wrap. The ends of a groove need to be closed to retain the axial tip seal. It is also desirable to close the ends of a tip seal groove to prevent the tip seal groove from becoming a conduit for fluid leakage. The radially outer end of scroll wraps generally have less radial thickness. The radial thickness of the wraps can be reduced at the radially outer end because only the inner flank surface of the wrap has to be machined to make sealing contact with a flank surface on the adjacent scroll wrap. The force exerted on the flanks of scroll wraps by fluid in the fluid pockets is generally less at the radially outer end of the wraps than it is on the wrap flanks near the center of the scrolls. Reduction of wrap thickness at the radially outer end where less strength is required reduces scroll weight, reduces scroll diameter and reduces the weight of balance weights required to balance orbital movement.
Axial tip seals create a drag which resists sliding along the surfaces of the end plates. This drag increases the drive torque required, increases tip seal and end plate surface wear, can brake wraps from end plates and causes heating. Heating of the tip seals and the scrolls can shorten the life of fluid displacement apparatus. Contact between end plates and the axial tips of scroll wraps also creates a drag that increases drive torque, heat generation and wear. The greater the area of surface contact, the greater the drag force, but the sealing effect does not increase proportionately.
Lubrication is commonly employed to reduce drag. Lubricants also reduce wear, increase tip seal life and improve sealing. Under some circumstances a thin film of lubricant can increase drag due to the shear strength of thin oil films.